A typical combustion system includes a furnace or combustion chamber for burning fuel and an air preheater for heating an air stream before being injected into the furnace. In many applications, the air preheater heats the air using exiting flue gas. When fuel is burned in the combustion system, various gas phase pollutants are generated that must be minimized. Examples of gas phase pollutants include, but are not limited to, Sulfur trioxide, Mercury, and other volatile elements, inorganic and organic compounds.
Sulfur trioxide (SO3) is formed in boilers by the oxidation of sulfur compounds present in the fuel. SO3 is very reactive and extremely hygroscopic. SO3 readily combines with water vapor to form a sulfuric acid aerosol. At moderate stack concentrations, these aerosols form visible stack plumes with potential environmental hazard issues. The formation of SO3 in a flue gas stream is dependent upon various factors: fuel composition and preparation, boiler design and operating conditions, and the presence of a selective catalytic reduction (SCR) system for NOx reduction. The interaction of SO3 with ash particles through the air preheater is dependent upon the concentration of SO3 in the gas phase, the size distribution and population of the ash particles, and the temperature gradients and heat-transfer surface characteristics within the air preheater.